In the prior art, metals (such as aluminum) and polycrystalline silicon (hereinafter referred to as "polysilicon") have been used as materials for interconnecting conductors in semiconductor devices. Polysilicon is often preferrable in many such applications because of its stability at high temperatures and because silicon dioxide can be deposited (through chemical vapor deposition processes) or thermally grown thereon. One of the undesirable features of polysilicon is its relatively high electrical resistance. In order to overcome this problem, polysilicon has been replaced for many of these applications with silicides of certain metals, such as molybdenum, tantalum, tungsten, rhodium and mixtures thereof. An article by V. L. Rideout entitled "Reducing the Sheet Resistance of Polysilicon Lines in Integrated Circuits" and published in IBM Technical Disclosure Bulletin, Volume 17, No. 6, November 1974, teaches the use of hafnium silicide as a conductive material in the place of polysilicon for the purpose of decreasing the resistance of these conductors. U.S. Pat. No. 4,180,596 issued on Dec. 25, 1979 to Billy L. Crowder and Stanley Zirinsky, entitled "Method for Providing a Metal Silicide Layer on a Substrate" and assigned to the present assignee, describes a method for providing a metal silicide on a semiconductor substrate, using a refractory metal, such as from the group consisting of molybdenum, tantalum, tungsten, rhodium and mixtures thereof.
In the prior art some conductive lines made of metals or polysilicon have been used as sources for doping materials for the formation of doped regions in semiconductor devices. For example, U.S. Pat. No. 3,601,888 entitled "Semiconductor Fabrication Technique and Devices Formed Thereby Utilizing a Doped Metal Conductor" and issued on Aug. 31, 1971 to W. E. Engeler et al. teaches the use of a metal conductor made of a material such as molybdenum, doped with a conductivity determining impurity, such as boron, for use as a diffusion source and then as an electrode for the diffused region. U.S. Pat. No. 3,887,993, entitled "Method of Making an Ohmic Contact With a Semiconductor Substrate" and issued on June 10, 1975 to T. Okada et al. also discloses the use of a metal, such as tungsten, platinum or molybdenum which is doped with a conductivity determining impurity such as arsenic, boron, phosphorous or antimony to form diffusion regions in semiconductor substrates. Using a heat treatment the doping material is diffused in the semiconductor regions while the metal reacts with the semiconductor regions to form low resistivity ohmic contacts. This patent further suggests the use of two doping materials of opposite conductivity type on different portions of a metal line to form N-type and P-type diffusions on different regions of the substrate during the same diffusion steps.
U.S. Pat. No. 4,364,166 entitled "Semiconductor Integrated Circuit Interconnections", issued on Dec. 21, 1982 to Crowder et al. and assigned to the present assignee, discloses inter alia, the use of a composite structure of polysilicon and a metal silicide (hereinafter referred to as "polycide") on a substrate, with the polysilicon of the polycide doped with a material that outdiffuses into predetermined regions of the substrate during a heating cycle to make them conductive.